Radio switch

[DennisRhodes]

I see planes with a "Radio" switch, switching between 2 radios, when there is no audio panel. I am wondering what is being switched. Is it just power? How is the audio and transmit functions being dealt with? Is it switching a relay that controls which radio is transmitting? Each radio has its own antenna, so either can be used, but with only one pilot headset, I am guessing it might control the PTT button to a specific radio, but then the mic inputs would need to be paralleled.....

Am I overcomplicating this?

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Generally the mike and PTT is switched on a Double pole , Double throw toggle switch. you would still have audio from both but could always turn the volume down on one . If both radios have an Tx light you can verify the switch position each time you Tx. Poor Mans audio panel BUT it does have good application in a small panel space AC.

 

[Flyingleap]

If two pilots.....need an audio panel to talk to each other. Maybe an old Intervox AA80-001, if want to be cheap.

 

[ShawnR]

Thanks for the replies. So, both radio audio outputs are simply paralleled into the headsets? The 9A I flew in recently had 2 radios wired in, albeit one was out for repair. The pilot and I communicated fine and the transmit function seemed to work, although we did not talk to anyone in particular as no one else was in the area that day. I assume it worked ok.

I have seen this set up more than once and wondered how or why. That is, till I priced audio panels. I don't plan on a second radio in the -4 other than a portable for backup, but was curious how it worked. The DPDT switch makes sense.

 

[BobTurner]

Thanks for the replies. So, both radio audio outputs are simply paralleled into the headsets? The 9A I flew in recently had 2 radios wired in, albeit one was out for repair. The pilot and I communicated fine and the transmit function seemed to work, although we did not talk to anyone in particular as no one else was in the area that day. I assume it worked ok.

I have seen this set up more than once and wondered how or why. That is, till I priced audio panels. I don't plan on a second radio in the -4 other than a portable for backup, but was curious how it worked. The DPDT switch makes sense.

Terminology: "audio panel" generally contains an intercom for pilot-copilot conversations, as well as switching/mixing of various avionics' audio output (#1 com, #2 com, VOR, ADF (?), etc.,) as well as switching mikes and PTT lines. An "intercom" may just have inter-pilot communications, or it may have extra inputs for mixing of various radios' outputs (but usually no switching). Just paralleling the outputs of multiple radios may not work: if the impedance looking back into the radio is small (it often is) then one radio will tend to "short out" a second if they are simply in parallel. This is properly fixed with an audio "mixer", but if there are only two, maybe three devices, they can be sufficiently isolated from each other by putting 220 ohm resistors in series with each output.

 

[Flyingleap]

Shawn, I think what you are talking about is a Garmin GTR 225 with:

• Built-in, two-place voice activated intercom

A single comm radio, built in intercom, single double throw switch, and a communication with the copilot.

Is that what you saw?

 

[Scott Hersha]

Thanks for the replies. So, both radio audio outputs are simply paralleled into the headsets? The 9A I flew in recently had 2 radios wired in, albeit one was out for repair. The pilot and I communicated fine and the transmit function seemed to work, although we did not talk to anyone in particular as no one else was in the area that day. I assume it worked ok.

I have seen this set up more than once and wondered how or why. That is, till I priced audio panels. I don't plan on a second radio in the -4 other than a portable for backup, but was curious how it worked. The DPDT switch makes sense.

I also didn’t want to buy an audio panel when I have only one radio (COM2 is my handheld also). I think if you have 2 COM radios, to get full functionality of both, you need an audio panel. On my solo GTR200B, with it’s built in intercom, I don’t need an audio panel. With 2 COM radios, maybe you should consider one so you can use both of them without canceling one of them out. If you are not installing an audio panel, then I think you would need a switch that switches both transmit and receive functions at the same time (including your PTT switch). Others have done this and it works, but you need two antennas, which apparently you have. I didn’t want another antenna either. My handheld is easily attached to my external antenna with a pigtail cable, making sure I turn OFF my COM1 so I don’t fry it without an antenna connected of I accidentally bump my PTT switch.

 

[ShawnR]

The plane has a Flightcom 403 intercom and this radio switch. I guess, as mentioned by DennisR, the switch controls which radio transmits. In on line documents I found for the 403, it does not say 2 radio inputs. It is a 9A I just purchased (days before winter hit here), but I cannot access it till snow melts so I am trying to figure out what might be going on in the panel. I am not able to speak with the builder. It has been flying since 2006 with no audio panel and I assume working the way it was intended. I just don't know what that was. Currently, one radio, Icom A200 is not working and not repairable. Looking ahead, I will want to replace the non working radio as I like that idea. And the carrier is already in the panel, but Icom said I will need to make some wiring changes to accommodate the A220. So, I am trying to figure out/guess how it might currently be wired. I am guessing that the audio is wired in parallel, which I am not a fan of for the reasons mentioned by Bob. I will look for a resistor, and try to follow out the wiring when the temperatures are somewhat not Arctic like (May maybe? )

I should add that, as Scott mentioned, this plane does have 2 antennas.

Thank you for all of the inputs.

 

[RV6_flyer]

Thanks for the replies. So, both radio audio outputs are simply paralleled into the headsets? The 9A I flew in recently had 2 radios wired in, albeit one was out for repair. The pilot and I communicated fine and the transmit function seemed to work, although we did not talk to anyone in particular as no one else was in the area that day. I assume it worked ok.

I have seen this set up more than once and wondered how or why. That is, till I priced audio panels. I don't plan on a second radio in the -4 other than a portable for backup, but was curious how it worked. The DPDT switch makes sense.

The audio out from both radios will use an audio summing circuit / chip that isolates the audio out from feeding back into the audio of the other radio. This can be done with an OpAmp or cheaply with a resistor in both audio outputs.

 

[ShawnR]

The audio out from both radios will use an audio summing circuit / chip that isolates the audio out from feeding back into the audio of the other radio. This can be done with an OpAmp or cheaply with a resistor in both audio outputs.

Thanks Gary. That is good to know what I might be looking for. I went through a very large box of documents that came with the plane yesterday and the only related item I found was the 403 brochure. Granted, I was just trying to organize the box. Now that it is somewhat sorted, I will go look for something related to what you and Bob suggest, a resistor or summing circuit box. I am pretty sure he would not have built a summing circuit but maybe there will be some note along those lines or notes on the wiring.

 

[RV6_flyer]

Thanks Gary. That is good to know what I might be looking for. I went through a very large box of documents that came with the plane yesterday and the only related item I found was the 403 brochure. Granted, I was just trying to organize the box. Now that it is somewhat sorted, I will go look for something related to what you and Bob suggest, a resistor or summing circuit box. I am pretty sure he would not have built a summing circuit but maybe there will be some note along those lines or notes on the wiring.

Shawn:

For the past 20-years, I have been using two radios with the toggle switch configuration with resistors in series with the audio out from each radio. I forget what value the resistor is.

 

[BobTurner]

Shawn:

For the past 20-years, I have been using two radios with the toggle switch configuration with resistors in series with the audio out from each radio. I forget what value the resistor is.

Any value around 220 - 470 ohms works fine.
BTW, I built my own active mixer, using one IC chip. It’s a very easy project. So you (the OP) might have one. They’re also available commercially (about $60 - $58 for labor, $2 for parts).

 

[ShawnR]

Any value around 220 - 470 ohms works fine.
BTW, I built my own active mixer, using one IC chip. It’s a very easy project. So you (the OP) might have one. They’re also available commercially (about $60 - $58 for labor, $2 for parts).

I suspect I will find resistors when I go looking

@BobTurner , what IC did you use for your active mixer? Sounds like an interesting project on these cold nights.

 

[BobTurner]

I used an LM324, designed for a single sided power supply. Google ‘audio mixer’ for circuits.

 

[ShawnR]

I used an LM324, designed for a single sided power supply. Google ‘audio mixer’ for circuits.

Thanks Bob. I do have those in stock.

 

[bbaggerman]

For an audio mixer go take a look at the nifty little unit available at MakerPlane.org.

2-Channel Mono 4-Channel Stereo Audio Mixer (AMX -10A)

MakerPlane Open Source Aviation. Electronics kits, instruments and parts.

store.makerplane.org

 

[flightlogic]

For an audio mixer go take a look at the nifty little unit available at MakerPlane.org.

2-Channel Mono 4-Channel Stereo Audio Mixer (AMX -10A)

MakerPlane Open Source Aviation. Electronics kits, instruments and parts.

store.makerplane.org

Bob, that's a lot of "stuff" crammed into the DB25 shell. Very impressive. In my day job I work with audio mixers that do 6 transmitters and 8 receivers...
(medical/law) and the prices would make your toes curl.
So, for someone to design a simpler audio isolation- mix for non certified is really a boon to builders.
Cheers